The present invention relates generally to finned tube heat exchangers, and more particularly to a finned tube for a heat exchanger having vortex generators on the fins thereof.
Most large-scale heat exchangers, such as the air-cooled condensers used in binary-cycle geothermal power plants, require the use of finned tubes in order to increase the heat transfer surface area. A finned tube in a heat exchanger is generally comprised of a tube with a series of fins extending from the outer surface of the tube along its length. Such fins may be plate-type individual fins or wound in a spiral-type configuration along the length of the tube. In a condenser such as an air-cooled condenser, coolant such as air is typically forced through several rows (or a xe2x80x9cbundlexe2x80x9d) of long, individually-finned tubes by large induced-draft fans or the like. The condenser units in a power plant can be very large and represent a significant percentage of the overall capital cost of the plant. In addition, the power required to operate the fans typically represents a significant parasitic house load, thereby reducing the net power production of the power plant. Therefore, it would be generally desirable to increase the heat transfer performance of the finned tubes without significantly increasing the cost of the condenser or the power required to operate the fans.
Generating counter-rotating longitudinal vortices in the fluid flow path along the finned tube periphery results in a more efficient exchange of heat. This is due at least in part to the fact that longitudinal vortices disrupt boundary layer formation and mix the fluid (e.g., air) stream near the fin and tube surfaces with the main fluid flow stream. Certain longitudinal vortices, called xe2x80x9chorseshoe vorticesxe2x80x9d, are generated naturally in finned tube heat exchanger passages by the interaction of the fluid flow with the curved surface of a heat exchanger tube. The heat transfer performance of finned tubes can be further improved by generating additional longitudinal vortices, which can be created through the use of vortex generators on the individual fins.
Vortex generators may be comprised of a series of winglets mounted on or punched into the fin surfaces. Depending on the shape of the winglets and the position of the winglets on the fins, heat transfer performance can be significantly improved with a minimal increase in pressure drop along the finned tube.
The present invention is directed to a method of manufacturing a finned tube for a heat exchanger. A continuous fin strip and a tube are provided. The tube has a wall with a continuous cross-sectional shape, an internal surface and an external surface. At least one pair of vortex generators is produced in the fin strip. This may be accomplished by punching at least one pair of winglets out of the continuous fin strip, thereby producing corresponding openings in the continuous fin strip. Each of the winglets has at least one folded edge such that it extends from a surface of the continuous fin strip adjacent to its corresponding opening. Concurrently with and subsequent to producing the vortex generators in the continuous fin strip, the tube is rotated and linearly displaced while the continuous fin strip with vortex generators is spirally wrapped around the external surface of the tube. This results in producing at least one pair of vortex generators on each 360-degree section of continuous fin strip.
The present invention is also directed to a system for manufacturing a finned tube for a heat exchanger. The system includes a continuous fin strip and a vortex generator die assembly operatively connected thereto. The vortex generator die assembly is adapted to produce at least one pair of vortex generators in the continuous fin strip, thereby creating a continuous fin strip with vortex generators. The vortex generator die assembly may comprise a male punch having at least one pair of tapered protrusions and a female die having at least one pair of indentations corresponding to and adapted to receive the protrusions of the male punch. The vortex generator die assembly is adapted to punch at least one pair of winglets out of the continuous fin strip, thereby producing corresponding openings in the continuous fin strip. Each of the winglets may have at least one folded edge such that each of the winglets extends generally perpendicularly from a front surface of the continuous fin strip adjacent to one of the corresponding openings. The system also includes a tube assembly having a tube holding device. Operatively connected to the tube holding device are a rotating device and a linear displacement device. A tube held by the tube holding device is rotated by the rotating device and linearly displaced by the linear displacement device while the continuous fin strip with vortex generators is spirally wrapped around the tube, thereby producing at least one pair of vortex generators on each 360-degree section of continuous fin strip.